Part One:
The Battle of the Coral Sea

Japanese Strategy

Japan started the war in Asia and in the Pacific to establish and protect a "New Order" in Asia.2 In "Phase I" of the war, the "operational objective" of the Imperial Fleet, n the words of Combined Fleet Secret Order Number One, issued on 1 November 1941, was expressed as follows: ". . . by ejecting British and American strength from the Netherlands Indies and the Philippines, to establish a policy of autonomous self-sufficiency and economic independence."3 Willmott, in the same vein, asserts that

. . . of all the major combatants Japan alone did not aspire to a final victory. . . . Japan's goal was to secure a negotiated peace by limiting and winning the conflict she began . . . in 1941. She aimed to force her enemies to come to terms with the gains she intended to make in the opening months of the war.4

Asia was defined by those Japanese who shared this vision of the immediate future as including India and Indonesia, as well as China, Manchuria, and all of Southeast Asia, including the Philippines. Australia was on the periphery of Asia, in danger of being swept into its definition at the next favorable turn of events.

Phase I of the Japanese master plan for the conquest of this vast area actually ended in the central and western Pacific in march 1942, with the unexpectedly quick and easy defeat of Australian, British, Dutch, and American forces and the fall of Java. Heroic American naval, ground, and air forces on Corregidor did not capitulate until 6 May 1942. The fall of Java, however, marked the end of effective naval resistance in the entire region from Singapore to New Guinea.

Flush with their uninterrupted string of victories, Japanese army and navy planners agreed, probably in late December 1941 or early January 1942, that the United States and Great Britain must be prevented from developing Australia as a base from which to launch a counteroffensive. How this ambitious goal was to be accomplished became a matter of contention, however, and a controversy developed between the army and navy over the "propriety" of actually invading Australia and India. The navy reasoned that, to keep the U.S. and Great Britain on the defensive, all Japanese military arms should be constantly on the offensive. Accordingly, naval strategists recommended a far-reaching but vastly unpopular menu of joint army/navy amphibious offensives throughout the central and western Pacific and in the Indian Ocean to be accomplished in the first six months of 1942.5

The navy wanted the army to invade India and Australia. Always mindful of its heavy troop commitments in China, Manchuria, and Southeast Asia, and never an enthusiastic supporter of naval strategy, the army managed to convince the navy that the strategy was


beyond its capabilities. Instead, it agreed to a policy also conceived in the navy, but much more economical of army resources, to help the navy sever communications between the U.S. and Australia and the U.K. and India. If successful, such a policy would complete the isolation of Australia, prevent an Anglo-American counteroffensive in the Pacific, reduce western aid to China, and place India, Australia, and New Zealand well within the sphere of Japanese influence without resorting to a serious expenditure of army resources. Thus, the army and navy created out of this compromise the basic Japanese strategy of Phase II of the war in Asia and the Pacific: to isolate and neutralize Australia and India, rather than invade them.6 The Japanese Navy lost no time in implementing this policy.

The Coral Sea and Surrounding Area

On 23 January 1942, six days after their successful operation against the city of Rabaul on the island of New Britain, Japanese Imperial Headquarters published Naval Directive #47, which immediately became the basic architecture for their Phase II strategy. It ordered the army and navy to cooperate in seizing the ports of Lae and Salamaua on the northeast coast of Papua/New Guinea.7 After these operations had been completed, the navy alone was to seize Tulagi, and island near Guadalcanal in the Solomons. Following the occupation of Tulagi, the army and navy were again to cooperate in seizing Port Moresby, an Australian base on the southeast coast of Papua/Mew Guinea. After Moresby was secured, the Japanese planned to launch their campaigns against New Caledonia, Fiji, and Samoa, all of which lie east of Australia, thus effectively encircling the northern half of eastern Australia and cutting off communications with her U.S. allies.8


In mid-February a "local agreement" was reached by the army and navy to implement Directive #47 in early March beginning with the joint effort to seize Port Moresby.9 To effectively isolate India, the main base for western aid to China, and incidentally to protect the army's efforts in Burma, the Combined Fleet also planned unilaterally to continue strategic offensive operations and to inflict as much damage as possible on the British Far Eastern Fleet by simultaneously sending carrier strike forces to the Bay of Bengal and the Indian Ocean.10 Immediately after the Moresby operation, Directive #47 ordered the Fourth Fleet to seize Ocean and Nauru in the Gilberts. These lightly defended equatorial islands were part of the Trust Territory of the Pacific administered by the United States. Then the Combined Fleet would regroup at Rabaul and Truk to carry out joint operations against New Caledonia, Fiji, and Samoa.11

In the euphoria of their victories, the Japanese disregarded the possibility that any power or weapon could derail their intentions. However, thanks to the aggressive U.S. strategy of using carrier task forces in hit-and-run raids, the road to Japanese success in Phase II was not to be as smooth and uninterrupted as in Phase I. Two episodes involving U.S. carrier aircraft convinced Admiral Shigeyoshi Inouye, Commander in Chief, Fourth Fleet (CINC 4), that control of the skies was not assured and the he needed carrier support to counter the unforeseen U.S. air capability if he were to succeed with his planned invasion of Port Moresby.12

American Countermeasures

For the United States, 1942 began with some extraordinary bad luck with respect to carrier strength. On 11 January 1942, less than two weeks after Admiral Chester W. Nimitz had assumed command of the Pacific Fleet, the carrier Saratoga was torpedoed by a Japanese submarine. Damage was so extensive that the vessel, only just arrived in the Pacific, was returned to the West Coast for repairs. This temporary loss reduced the U.S. Pacific Fleet o three fast one-carrier task forces (TFs): TF 8, Enterprise, commanded by Captain George D. Murray; TF 11/16, Lexington, commanded by Rear Admiral Aubrey W. Fitch; and TF 17, Yorktown, commanded by Vice Admiral Frank Jack Fletcher. TF 17 first escorted reinforcements to Samoa and became available for offensive operations only on 23 January 1942, the same day Rabaul fell to the Japanese.13

Regardless of the Pacific Fleet's reduced circumstances, however, Admiral Ernest J. King, Commander in Chief, U.S. Fleet (COMINCH), insisted on a more aggressive plan of attack during the early months of 1942.14 From the intelligence reports provided by the Pacific radio intelligence (RI) centers, he was confident of the success of the proposed hit-and-run ventures. He knew that the Japanese had weakened their defenses in the central Pacific by transfers of land-based aircraft from the Mandates to the southwest Pacific, and he knew in detail the whereabouts and often the destinations of each element of the Japanese Combined Fleet.


USS Enterprise

On 1 February 1942, Admiral Nimitz dispatched two one-carrier task forces, Enterprise (TF 8) and Yorktown (TF 17), to conduct raids on Japanese-held islands west of Hawaii. The Enterprise operated against the Marshalls, Wake, and Marcus Islands until joining the Hornet, commanded by Captain mar A. Mitscher, out of San Francisco, for the raid on Tokyo in April. The Yorktown also attacked the Marshalls in early February but then saw little action until early March15 During those early months of 1942, the U.S. Navy also accomplished a major feat in establishing and maintaining air superiority using carrier aircraft over a small but critical area in the northern Coral Sea.

In mid-February, Admiral King temporarily transferred the Lexington to Admiral Herbert F. Leary, head of ANZAC naval forces with headquarters in Australia. Admiral Leary planned with Admiral Wilson Brown, commander of the task force, to send the Lexington immediately against he Japanese base at Rabaul. This plan was put into action, but on 20 February TF 11 was discovered before it could deliver its attack. For


USS Hornet

USS Yorktown


several hours that day the Lexington participated in the first battle of the young war between American fighter and Japanese bomber aircraft about 300 miles east of Rabaul. It was a signal American victory. Through a minor skirmish by later standards, it seriously disrupted the balance of air power in the region. The Lexington lost only two planes and one pilot while the Japanese lost at least eighteen planes and pilots. The TF 11 engagement, not decisive in itself, inflicted serious losses in aircraft and pilots at a rate that Japan could not possibly sustain.16

Despite the success of TF 11, it was clear that the U.S. and its allies were not seriously impeding the Japanese war machine in the southwest Pacific. By 9 March 1942, when MacArthur was completing arrangements to leave Corregidor, the Japanese controlled the Netherlands East Indies and virtually the entire north coast of Papua/New Guinea. On 8 march, Lae and Salamaua fell to the Japanese, and Rabaul-based aircraft were poised to strike at Port Moresby on the south coast of Papua and Finschafen on the north coast near Lae. These strong points were the last significant obstacles to complete Japanese domination of the world's second largest island. Unknown to the Japanese, however, another event involving a U.S. carrier task force was about to cause a major convulsion in their plans.

After the aborted raid on Rabaul, Admiral Leary returned the Lexington to CINCPAC control. On 10 March 1942, while the Japanese were savoring their victories at Lae and Salamaua and were preoccupied with seizing the nearby port of Finschafen, the Lexington and the Yorktown, now constituting a new TF 11, lay undetected off Port Moresby. The task force launched its planes against the Japanese forces at Lae and Salamaua, attacking through a 7,500-foot pass in the 15,000-foot Owen Stanley Mountains, guided to their destination by a single carrier plane, which orbited over the pass until the attack was completed. Compared to the engagement with land-based bombers off Rabaul on 20 February, the American air raid on 10 March did little actual military damage.17 But it once again demonstrated to Admiral Inouye that he probably did not enjoy the necessary control of the skies required for the planned attacks on Port Moresby and Tulagi.


Japanese Preparations for Moresby

The original Japanese plan called for an amphibious assault on Moresby, Operation MO, in March 1942 to be supported only by land-based bombers from Rabaul and seaplanes from newly constructed bases, i.e., Lae, Salamaua and Finschafen. With the feasibility of this plan in jeopardy because of the presence of an effective U.S. carrier force, Admiral Inouye sought to postpone the invasion of Moresby. He turned to his superior, Admiral Isoroku Yamamoto, Commander in Chief, Combined Fleet, with a request for carrier support to ensure Japanese air superiority before proceeding with the invasion of Moresby.

Admiral Yamamoto agreed with Admiral Inouye's assessment and allowed him to postpone Operation MO until May.18 Yamamoto also agreed to provide the carrier Kaga from Carrier Division (CarDiv) 119 supplemented by a recently commissioned light carrier, the Shoho, which was not yet assigned to a division.20 The Kaga, however, was reported torpedoed on 17 March in the Lombok Straits.21 This development led to the substitution of CarDiv 5's two carriers, Shokaku and Zuikaku, to the Fourth Fleet for the first two weeks of May.22 These changes seriously disrupted Yamamoto's original timetable for Phase II.

Japanese Aircraft Carrier Kaga


Japanese Aircraft Carrier Shokaku

Japanese Aircraft Carrier Zuikaku


Following the decision to postpone the invasion of Moresby, Tulagi, Ocean, and Nauru Islands until a carrier force became available, Admiral Inouye deployed his forces in areas that were relatively secure from the threat of American harassment. Using four heavy and two light cruisers to cover three destroyers, two gunboats and the light carrier Chitose, CINC 4 seized islands between 10 March and 19 April as far eastward as the Shortlands and constructed bases on the north coast of New Guinea, impressively completing a defensive perimeter from the Solomons to the Philippines.23

On 23 April, Admiral Inouye issued South Seas Force Order #13 to resume operations against Port Moresby. Analysts at HYPO, the radio intelligence center at Pearl Harbor, speculated that this order was actually delivered by plane from CINC 4's Truk headquarters rather than by radio communications.24 With the carriers and their escorts, the combined forces involved represented one of the largest collections of ships ever assembled by the Japanese Navy.25 The Combined Fleet, meanwhile, increased its efforts to build up ground and air forces at Rabaul and in the Mandates in anticipation of the larger Phase II operations against Fiji, Samoa, and New Caledonia. The Shoho and numerous cargo vessels were used to ferry land-based fighter and bomber aircraft from Japan through the Mandates to Rabaul while the airfields on Timor, New Britain, and New Guinea continued to conduct air raids on Darwin, Moresby, and bases and towns in the Solomons. Many of these Japanese preparations for their numerous Phase II operations were reflected in their naval communications and were detected by U.S. Navy communications monitors located at the radio intelligence centers in the Pacific Basin.

Naval COMINT Centers and Their Reports on Japanese Preparations for the Invasion of Port Moresby, Fiji, Samoa, and New Caledonia

What time is it? According to current international standard time charts, the region between eastern Australia and Bougainville is in the Z=1- or K time zone. At the beginning of the war, however, all U.S. clocks were set on War Time (+1 hour). This explains why, in HYPO and Melbourne messages and in this narrative, all local times in this region are treated as Z+11. An occasional discrepancy of thirty minutes appears in the Yorktown intercept log because the ship sailed into and out of a +/- 30-minute time difference.

May is the third month of autumn in this region. According to Time Books World Weather Guide, which National Geographic consulted at my request, daylight in the Coral Sea at 10 degrees south on 15 May lasts 11 hours, 40 minutes, and at 20 degrees south, 11 hours 14 minutes. Adding a few minutes for false dawn and twilight and a few minutes for 7-8 days' difference in date when the days would be longer, and allowing for the fact that all the action directly involving Admiral Fletcher took place at approximately 15 degrees south, the hours of visibility on 7 and 8 May 1942 were probably 0700 to 1900 local time.

By the middle of March 1942, two viable naval radio intelligence centers existed in the Pacific: one in Melbourne, Australia, and one, HYPO, in Pearl Harbor, Hawaii. In response


to the deteriorating situation on Corregidor, a possible threat against Hawaii, and the demand for information from everyone in official Washington, another center (NEGAT) was formed in Washington in February 1942 by the Communications Directorate using elements of OP-20-G. The center on Corregidor (CAST) was no longer affiliated with a fleet command, and its collection and processing capabilities were rapidly disintegrating as a result of evacuations of personnel to Australia and destruction of its facilities by bombing and gunfire. Prior to March, however, its contributions to the rapid advances being made in naval cryptanalysis by the United States Navy were substantial. This was possible because, almost immediately after the war began, all the centers were linked by much improved communications, which made possible the rapid exchange of cryptographic information.

OP-20-G   Cdr. John R. Redman
GA   Cdr. Joseph N. Wenger
GB Correspondence Howes
GC Communications White
GD Deception Smith
GF DF Net-East Coast LCdr. Scott
GI Combat Intelligence Sam Bertolet
GL Collateral Information LCdr. A.W. Kramer*
GM Math Analysis Research Cdr. H.T. Engstrom
GP Planning and Equipment Stanier
GR Training and Personnel Hayes
GS Machine Processing LCdr. H.O. Hogan
GT Traffic Analysis Lt. R.I.F. Fravel
GW Traffic Receiving LCdr. W.F. Harrington
GX DF and Intercept Control LCdr. Welker
GY Cryptanalysis and Decryption LCdr. Parke
GZ Translation and Code Recovery Kramer *
  * Same Person
OP-20-G Washington (NEGAT) Organization, February-June 1942

For some time, OP-20-G had used air mail for the transport of vital cryptographic material. In early January 1942, OP-20-G initiated a desperate attempt to speed up delivery of current cryptographic recoveries to each center. Washington authorized each center to send code and additive recoveries in radio messages on a special "COPEK" channel that linked CAST to HYPO and Washington, and that, by the end of March, included Melbourne. The organization continued to use surface mail for the basic traffic.26 Pearl Harbor and Melbourne shared the same Japanese Navy targets, while CAST, for the remainder of its existence, reverted to priority support for the local commanders. Melbourne from the outset, however, was a joint Australian-U.S. Navy effort supporting an international and unified command. Admiral Haret mandated this relationship when the navy hastily moved Station CAST from the Philippines to Melbourne, Australia.



The Station CAST contingent in Melbourne quickly evolved into the dominant partner. This was partly because of the central role played by the American forces in the southwest Pacific as well as the unique radio intelligence capability represented by the CAST evacuees. Lieutenant Rudolph Fabian, the officer in charge (OIC), who had led the first party "deployed" from Corregidor, immediately established a collection and processing effort in Australia. Using almost exclusively his own resources, Fabian began to produce information concerning the Japanese fleet and its activities with particular emphasis on those elements of the Japanese fleet that threatened Allied naval forces in the southwest Pacific.

He also provided daily briefings for Admiral Herbert F. Leary and other senior U.S. Navy officials in the ANZAC forces. MacArthur's headquarters soon discovered his presence in Melbourne, and Admiral Leary ordered Fabian to include the Supreme Commander, Southwest Pacific Area (COMSOWESPACFOR). Despite its obvious strategic and tactical value, there is strong evidence, to be introduced later, that MacArthur, who was briefed in private, seldom used Fabian's information during this period to support the naval task forces under his command. (Moreover, the evidence that follows suggests that, when it was used, it was likely to be misused.) This pattern of daily briefings continued until MacArthur's headquarters moved to Brisbane in the fall of 1942.27 Because of the disappearance of the organizational element known as the 16th Naval District and the substitution of ANZAC headquarters in its place, the situation in Hawaii regarding command relationships and command intelligence interests was completely different.

GO Diplomatic and Naval Attaché Systems
GX Traffic Intelligence Division (3 officers, 4CRM)
    Japanese Weather Codes
    DF Plotting Center
    Traffic Analysis
GZ Research and Exploitation of Codes (8 officers, 3 men from Band)
GY-1 Code and Cipher Identification and Key Recovery (JN 25)
GY-2 Cryptanalysis of JN 11
GY-3 Cryptanalysis of JN 33, 40, 41, 42, 108, 155, 166, 180, 182, 183, and 199
GY-5 Translations of all messages and routing of CI summaries, originator of crypto-status reports
COM 14 (HYPO RI Unit, January-June 1942


Hawaii (HYPO)

In Hawaii, the radio intelligence center known as HYPO was administratively subordinate to the 14th Naval District, just as Station CAST in the Philippines before the war had been subordinate to the 16th Naval District. HYPO was commanded by Commander Joseph J. Rochefort, who was both a Japanese linguist and a cryptanalyst trained in OP-20-G by Laurance Safford and Agnes Driscoll. Highly regarded by Safford, Rochefort at one point was the head of OP-20-G while Safford satisfied his obligatory period of sea duty. Rochefort and Commander Edwin T. Layton, the intelligence officer for the Pacific Fleet, who was also a Safford-trained cryptanalyst, enjoyed an exceptional working relationship and a strong friendship. It has begun years before when they were both language students in Japan. Their offices were connected by a special telephone over which they conversed, often several times each day.

Rear Admiral Edwin T. Layton

Lieutenant Joseph John Rochefort

Lacking such a close working relationship with anyone in the 14th Naval District, which had no intelligence requirements to which it could conceivably respond, it was natural for Rochefort to consider CINCPAC as his command rather than the District. Slipping into this error was probably easy for Rochefort because virtually all of HYPO's resources were devoted directly or indirectly to satisfying intelligence requirements that originated with CINCPAC. Moreover, Bloch, who had preceded Kimmel as CINCPAC, never required an accounting from Rochefort.


1918-1925 Fleet/School
1925-1929 OP-20-G
1929-1932 Japanese Language Training
1932-1933 ONI
1933-1936 Fleet (USS Maryland)
1036-1938 11th Naval District (ONI)
1938-1939 Fleet (USS New Orleans)
1939-1941 Fleet (USS Indianapolis/ Staff
Scouting Force (US Pacific Fleet)
1941-1942 OIC Com 14 RI Unit

Before 7 December 1941, intelligence requirements that controlled the collection, processing and reporting activities of the Hawaiian RI center were not stated in a formal manner by the Commander in Chief, Pacific Fleet. HYPO followed a simple collection and processing logic to the complete satisfaction of Layton and Admiral Kimmel. Its goal was to detect and report threats to the Pacific Fleet by monitoring the communications of the Japanese fleet. It gave priority to recovering communications of the Japanese Navy shore establishment, of major Japanese fleet units, especially battleships, cruisers, and carriers, and of those related to local developments, particularly those involving submarines. From the surviving daily summaries from Com 14 and the CINCPAC Intelligence Staff, it is clear that they were also concerned with ground-based air units, particularly those deployed outside Japan. This latter concern did not arise entirely from the threat to U.S. possessions posed by the locations of the air units. The deployments of air units were also viewed a harbingers of offensive operations.

Between July and December 1941, HYPO traffic analysts Lieutenant Thomas A. Huggins and Lieutenant John A. Williams, no doubt encouraged by Rochefort and Layton, had freely expressed their opinions and time after time, through Rochefort, issued warnings of Japanese intentions. It was commonplace for them to assign a possible meaning or perspective to the patterns of fleet communications that might indicate concentrations of various elements, particularly if they threatened an important place of simply suggested hostile Japanese fleet intentions in general. The insights Rochefort and his analysts provided were usually forwarded to Admiral Kimmel unchanged by Commander Layton.28

Following the Japanese attacks on Pearl Harbor and the Philippines, however, HYPO reporting lapsed briefly into an eclipse that lasted until late January 1942. In sharp contrast to the incisive analytical reports that seldom left a Japanese initiative unaccounted for, probing beneath ship movements and patterns of communications


activity, HYPO reports for weeks after 7 December 1941 were usually empty of any insight or interpretive comments. The daily reports continued to reflect comprehensive coverage of communications activity and fleet movements, but the bare facts were offered without any commentary whatever, leaving the reader to decide what it all meant. They also lacked something that had formerly been the hallmark of HYPO's analytic capability: a willingness to publish warnings whenever appropriate. one noteworthy example of this deficiency occurred in February 1942. Between 17 and 19 February, HYPO followed CarDivs 1 and 2, of the Pearl Harbor Strike Force, in the Netherlands East Indies, reporting their whereabouts each day. Layton's daily reports, perhaps lacking the insight formerly provided by the HYPO analysts, suggested that they represented a threat to Java and Timor. Sadly, this was not the complete story. On 19 February, without warning, planes from these carriers struck Darwin, Australia, in a devastating attack.29

Several reasons for the remarkable change in HYPO's approach to reporting on the activities of the Japanese Navy after 7 December 1941 are suggested by the recollections of two of the principal figures in Headquarters CINCPAC and HYPO at this time: Admiral Edwin T. Layton, intelligence officer to both Kimmel and Nimitz, and Captain Thomas Dyer, cryptanalyst and Rochefort's second in command at HYPO. Layton suggested that an explanation lay in the new intelligence requirements mandated by Admiral Nimitz soon after he became Commander in Chief, Pacific Fleet, on 1 January 1942. Almost immediately after Nimitz replaced Admiral William Pye (Pye relieved Kimmel on 17 December 1941), two circumstances combined to require a major change in intelligence priorities. One was the loss of Wake; the other was pressure from Admiral Ernest J. King, newly appointed Commander in Chief of the U.S. Navy.

Admiral King believed he understood the intent behind the Japanese Navy's activities. Instead of continuing Admiral Pye's conservative defensive policies, King wanted the Pacific Fleet to conduct aggressive carrier raids on Japanese bases in the Gilberts and the Marshalls to check the Japanese advance and, most importantly, to reduce the growing threat to Samoa, Fiji, and the US.-Australian supply routes. His orders left no room for Nimitz to pursue the cautious policies of his predecessor.30 Kings' attitude may well have influenced Nimitz to structure new and much more formal intelligence requirements than the informal understanding that prevailed before his arrival.31

Much more formal and specific than in the past, the new requirements called for information concerning "deployment of enemy carrier strike forces/other carriers; disposition and strength of the Fourth and Sixth Fleets; deployments and strength of the twenty-four shore-based air forces, particularly the 24th Air Squadron known to be in the Marshalls; and enemy forces capable of furnishing significant reinforcements to the Marshalls/Gilberts by late January 1942"32 Marked by the detachment of a commander who was unfamiliar with HYPO's capacity for analysis, the new requirements seemed to


call simply for data on Japanese Navy capabilities -- not warning, not insight into Japanese intentions, not even analysis. The new approach represented an immense departure from the past and indicated that the symbiotic relationship between HYPO and the CINCPAC intelligence staff had ended. It would be almost reassuring to end the explanation here, but Captain Dyer offered another possible explanation for HYPO's temporary metamorphosis.

Captain Dyer suggested that the unfortunate change in HYPO's approach to reporting on the activities of the Japanese Navy was caused by something far less tangible: HYPO suffered from a sense of guilt over what the world labeled as a spectacular failure of both intelligence and leadership.33 According to Dyer, all at HYPO who were associated with the catastrophe of 7 December 1941 vowed that such a surprise would never happen again. Apparently they decided to let the facts speak for themselves.

Though unlikely to represent a major cause for the temporary diminution of HYPO's genius, it was in fact literally weeks before the analytic talent at HYPO, whose output had shone so brightly in October and November 1941, once again reclaimed its role as a valued source of strategic intelligence to new commanders in the Pacific and Washington. Fortunately, the wake-up call for Rochefort and Layton to resume functioning as a team came stuttering to life in late January 1942, just as the Japanese Navy launched its Phase II offensive against Port Moresby.

OP-20-G Washington/NEGAT

Prior to 7 December 1941, OP-20-G's "intelligence" support to the Chief of Naval Operations, Admiral Harold R. Stark, was very limited. It consisted of support to the Atlantic direction finding (DF) net and the decryption and translation of Japanese diplomatic messages. By taking resources from OP-20-GY, the cryptanalysis organization, OP-20-G initiated a daily summary of information about the Japanese Navy on 14 December 1941. It was intended to satisfy the insatiable demand in Washington for current intelligence. It was an unsatisfactory solution to the task at hand because it contained second-hand information taken from the daily summaries from HYPO and CAST.34

In February 1942, OP-20-G completely reorganized, and its mission as a purely research facility abruptly changed. In the new structure, an analytic element called "Combat Intelligence" was created and designated OP-20-GI.35 This element produced little original current intelligence until March, when intercept was obtained from Cheltenham, Maryland, and current decrypts/translations on JN 25 (the Japanese Navy General-Purpose Code) messages began to flow from OP-20-GY and GZ, respectively. Tasked to disseminate the translations to the commands and field elements along with its own analytic reports, OP-20-GI became, in effect, a nominal COMINT center in Washington. Until 15 May, however, it continued mainly to publish translations and sporadic reports


based on the output of Hawaii, Corregidor, and Melbourne. On that date, the Communications Directorate authorized OP-20-GI to issue RI/CI summaries twice each day at 0600 and 1400, including traffic analysis (CI) reports prepared by the analysts in OP-20-GT.36 This development was important in following Japanese preparations for the assault on Midway. Except for its substantial cryptanalytic contributions between December 1941 and May 1942, however, NEGAT was not a functioning COMINT center on the order of Hawaii, Melbourne, and Corregidor when the Japanese began to implement their Phase II policies. Significant functions such as traffic analysis and the production of reports based on the work of resident analysts were not developed in Washington until after the Battle of the Coral Sea.

Communications Intelligence and Japanese Intentions Toward Port Moresby

On 29 January 1942, the same day that Japanese Imperial Headquarters ordered Admiral Inouye to seize Port Moresby in early march, messages containing upper air observations for the region to the south began to emanate from Rabaul. In February, Japanese land-based aircraft from Rabaul initiated bombing attacks on the port, the town, and the nearby airfield. As each of these activities was detected by COMINT, U.S. Navy communications intelligence centers in Hawaii, Corregidor, and Melbourne seized the opportunity to issue warnings to King, Nimitz, and Hart. After the formation of ANZAC, Admiral Leary also received warnings of Japanese "future operations" in the direction of "Lae, Port Moresby, and the Solomons."37 Collectively, the warnings led Layton (and Admiral Nimitz) to agree in late February that a Japanese offensive was planned for the Moresby area.38 Within a week, the task forces were also alerted.

Initially excluded by the principle of need-to-know from distribution of communications intelligence, even that published by Layton, the carrier task force (CTF) commanders were added as soon as their forays began to produce a Japanese reaction. Individual intelligence items based on COMINT were supplied through CINCPAC to his subordinates in a variety of intelligence media. From 1 to 16 March 1942, for example, a daily CINCPAC Combat Intelligence Bulletin39 based on COMINT supplied mainly from Hawaii, but with occasional contributions from Corregidor, was sent as a message to all CTFs. It was replaced on 17 March by the CINCPAC Intelligence Bulletin40 that was also based on COMINT with contributions from Melbourne and that, according to surviving records, was sent to the same distribution until 1 June, when it too ended temporarily.

After the middle of March, all CINCPAC Bulletins that contained intelligence concerning the Japanese fleet were based principally on the following sources: individual messages translated by the centers and sent to CINCPAC; COMINT reports usually but not necessarily, based on translations that originated each day within HYPO;41 the Com 14 Radio Digest that was usually sent onward by CINCPAC almost verbatim;42 and the Com 14 COMINT Summary.43 After 20 March, when Melbourne began its daily reports, HYPO


As an almost inflexible rule, the COMINT centers were not in direct contact with the task force commanders. Exceptions occurred only twice, both during the Battle of the Coral Sea. The two surviving messages summarized unique communications activity of the Japanese strike forces. Though not vital information -- the second message pertained to activity that had occurred over nine hours before -- these were the only two messages sent by Com 14 to the task force commanders during the battle.44 Even during a tactical crisis, Com 14's reports were sent only to CINCPAC and other COMB and COPEK addresses, i.e., COMINCH, COMSOWESPAC, and other centers via priority messages.

The surviving CINCPAC Message File for the period between March and the end of May 1942 suggests that MacArthur did not rely on COMINT. It also reveals that he too did not permit Fabian to communicate directly with the task force commanders. The messages sent to the task force commanders and to CINCPAC indicate that MacArthur's preference was clearly slanted toward visual reconnaissance, including both aerial and coast watcher sources; he seldom passed on to the CTFs any COMINT learned from Fabian. The same record also reveals that, after the Battle of the Coral Sea, COMINT almost never appeared in the messages from MacArthur's headquarters.45

MacArthur's apparent reluctance to use communications intelligence may be due in part to an extraordinary situation that developed within the first Corregidor contingent soon after Fabian arrived in Australia. For the first ten days between 20 and 30 March 1942, the OIC of the Melbourne center was not Fabian but Lieutenant Commander Redfield Mason, a Japanese linguist normally employed by the Office of Naval Intelligence (ONI) and formerly on the intelligence staff of the U.S. Asiatic Fleet. He probably occupied that position in the Melbourne center because of his rank, which was senior to Fabian's' and Leary's mistaken notion of Mason's role in the detachment when he arrived in Australia via Java with the first party to be evacuated from Corregidor. At the end of the first report prepared by Fabian and his men, Mason appended the following comment, which revealed his distrust of any COMINT report not based on a translation:

It should always be borne in mind that RI is subject to the errors induced by enemy deception at which they are quite adept, and to those resulting from the analyzers' misinterpretations of honest traffic. Hence RI, unconfirmed by CI or DF bearings, may give an entirely misleading picture.

This note was addressed "Memorandum for COMANZACFOR," Admiral Leary, but because of the confusion over subordination that briefly prevailed immediately after 17 March 1942, when General MacArthur arrived in Australia, the note was probably intended for MacArthur as well.

To mitigate the profoundly negative effects such a comment would undoubtedly have on any headquarters receiving the subsequent COMINT reports not based on translations, as well as on his own work force, Fabian appended a second note placed a more reassuring perspective on his men and their work:


Note #2: The information-gaining capabilities of the unit are somewhat restricted at present but, while we are bringing our records up to date, developing microfilm books, completing arrangements for moving, collaboration, etc., we are scanning what traffic is available to us with particular stress on that addressed to or originated by units suspected of being in the Bismarck Archipelago area and also watching for those units on the DF bearings.46

By this statement, Fabian, who was himself a cryptanalyst,47 cleverly defended an American process of communications analysis, probably keenly aware that for over twenty years his British counterparts had considered traffic analysis a "most valuable adjunct to" and a "trustworthy substitute" for cryptography.48 Soon after these notes appeared, Lieutenant Fabian replaced Mason as OIC, and Mason departed for Washington, where he became head of OP-20-GZ (Translations), a job usually held by a linguist appointed by ONI.49

It was a curious coincidence that this episode occurred just as the floodgates were about to open allowing a torrent of Japanese Navy messages to be translated by the U.S. Navy linguists at the Pacific centers. A major intelligence breathrough for the United States occurred early in February 1942, when U.S. Navy cryptanalysts discovered that the change in the cipher for the Japanese Naval General-Purpose Code (JN 25) that occurred on 4 December 1941, i.e., the introduction of cipher Baker 8, was in fact only a slight modification in the keying method employed in the Baker 7 cipher. To the consternation of the cryptanalysts at the time, however, the change from Baker 7 to Baker frustrated a promising series of analytic successes against the cipher.50 Successful penetration of the new cipher in February 1942 meant that all the work done since before Pearl harbor on code recoveries and on the Baker 8 cipher could be applied to the JN 25 messages intercepted until the cipher changed again. Capitalizing on the breakthrough by increasing the numbers of key personnel working on the Baker 8 cipher and JN 25 code recoveries, the U.S. Navy, within a month, gained the ability to read all intercepted messages sent in this enciphered code.

The increasing number of navy personnel assigned to the Pacific war effort in Washington and to the analytic centers in the Pacific slowly revealed to the commanders of defending U.S. forces the Japanese Navy plans for offensive action from the Aleutians to Moresby.51 Almost everything written in radio intelligence reports based on message traffic from mid-March to the end of May 1942 came from the one system, JN 25. By mid-April, Japanese messages were being intercepted, decrypted, translated, reenciphered and disseminated by HYPO within six hours of their original transmission!52 These reports would prove to be the most vital of the entire war.53

With the advent of readable messages, traffic analysts as well as cryptanalysts tapped a veritable cornucopia of Japanese Navy activities on which to report. Exuberantly, all of the centers routinely shared the full and often partially recovered text of JN 25 messages


with the commands they supported. The practice of providing partial message texts lasted well into the fall of 1942.54 Although there is no indication that erroneous decisions were made based on partial message texts, this practice often left the task of extracting meaning from a partial translation to the commander's intelligence staff.55 Notwithstanding this minor shortcoming, as will be seen in the following review of specific COMINT contributions to the decision-making processes of the headquarters served, all of the centers as well as the CINCPAC intelligence staff56 were prolific in producing COMINT reports that bore specifically on events occurring the area between Japan and Australia during the period 15 March-May 1942.

COMINT reports identified Rabaul with its concentrations of land-based bombers, Truk from which CINC 4 guided his surface raiders and submarines before moving to Rabaul, and the flagship of Admiral Yamamoto, CINC Combined Fleet, as the principal sources of much of the Japanese mischief in the South Pacific. The centers also recorded a buildup of air, subsurface, and surface forces in the Fourth Fleet, while the attacks by bombers from Japanese bases in the Bismarcks and Solomons on northern and eastern Australian coastal towns and Port Moresby continued with their attendant losses in Japanese aircraft and crews.

The air buildup in the Mandates and in the Rabaul area reached the point in April 1942 that Layton, affirming the view expressed in January by Admiral King, characterized it as a threat to U.S. supply lines to Australia. Adding possible substance to Layton's warming, HYPO on the same day reported without comment that the Japanese navy minister, in one twenty-four-hour period, addressed no less than eleven dispatches to Combined Air Force units in the Fourth Fleet area.57 Although HYPO drew no conclusions of warlike intentions from the navy minister's activity, the center in Washington EGAT) saw part of the air buildup as indicating a likely Japanese offensive emanating from Rabaul and published a warning report to this effect.58

In addition to the continuing weather reporting and bombing attacks in January and February, other revealing features to Japanese communications pointed unmistakably toward Japanese fleet objectives. Perhaps the most prominent features were the digraph/trigraph designator systems used throughout the Japanese fleet. The designators represented specific places throughout the Pacific and were often adapted to convey organizational information. The initial digraph/trigraph designators observed by navy COMINT analysts, though complex, were solved almost immediately. Digraphs beginning with A applied to American targets in the central and northern Pacific.59 Australian targets in the Papua/Solomons region were assigned designators beginning with R, and those beginning with D were British/Australian targets in the Indian Ocean (including Darwin: DP). From its initial discovery, COMINT analysts used this system as a basis for identifying Japanese Navy objectives.

On 23 March 1942, for example, less than two weeks after the Pacific centers published their first translations, Corregidor published a report containing recoveries for eighty-nine designators, including AF for Midway and RZQ for Port Moresby.60 In early April the designator RZP appeared in a variety of contexts, which added to a growing


conviction that it too represented Moresby. A common topic of the RZP messages concerned participation of the Kaga61 in the "RZP Campaign."62 Neither Com 14 nor Corregidor, which also noted the RZP/Kaga connection, hesitated to correlate RZP with Port Moresby.

The Japanese designator system also provided additional evidence that further Japanese planning for the Moresby campaign was not postponed by changing the initial planned invasion date. While the Fourth Fleet consolidated its hold on northern New Guinea and two weeks before CINC 4 issued South Seas Force Order #13, a message was intercepted and translated that revealed the order of battle (OB) for a Moresby task force that included carriers.63 Following publication of this message, Layton issued a warning that "a Japanese offensive into the Solomons, Ellice or Gilbert Islands" seemed likely.64 Ironically, the warnings were not accepted in the southwest Pacific.

From its arrival in Australia on 17 March 1942, MacArthur's headquarters regularly received all messages in the COMB and COPEK address groups, insuring that MacArthur and his staff received all warnings from Hawaii, Melbourne, Corregidor, and Washington. On the scene for only a month, MacArthur's intelligence staff did not believe the navy's reports about Japanese intentions against Moresby. In sharp contrast to the confidence COMINT inspired in U.S. Navy circles, particularly in Nimitz and Layton, the views of MacArthur and his staff reflected a curiously skeptical attitude.

On 21 April, Colonel Charles A. Willoughby, MacArthur's assistant chief of staff for intelligence, prepared a status report for the chief of staff, Lieutenant General R.K. Sutherland, based on, but not attributed to, COMINT from Melbourne.65 Willoughby's report questioned the prevailing navy wisdom regarding the meaning of the reported buildup in Japanese naval and ground-based air strength in the Fourth Fleet area. He declared that a buildup of Japanese sea power posed more of a threat to the coast of Australia and to New Caledonia than to Port Moresby.66 His view revealed not only selective reading of the daily reports from Melbourne and Hawaii but an alarming lack of appreciation for the complexities of air, naval, and amphibious warfare. Willoughby also claimed that Moresby was more vulnerable to attack by nearby land-based air unit and that aa successful attack would not require the carrier task force destined for the Fourth Fleet. (His lack of knowledge of amphibious warfare is understandable since few military planners in the U.S. had any experience in amphibious warfare at this stage of the war. Though amphibious warfare has extremely ancient origins, it seems that each generation has been forced to learn of it the hard way, through experience.)

MacArthur and Sutherland did not completely disregard the warnings from Fabian in Melbourne, but they were probably secure in the knowledge that their virtually unmolested air patrols into the Coral Sea would soon confirm the presence, or absence, of a Japanese amphibious force. Searching beyond Tulagi, the easternmost point attainable, however, was another matter.67 On both 25 and 27 April, General MacArthur warned the army commander on New Caledonia, who was far beyond the range of his aerial


reconnaissance patrols, of a likely Japanese attack. Unfortunately ,one and possibly both of the messages came to the attention of a newspaper correspondent. The controversy generated by these disclosures did little to reduce Allied anxiety over possible Japanese suspicion that U.S. analysts were reading their most secret communications.68

On 30 April, MacArthur received a message from General George C. Marshall, Chief of Staff, U.S. Army, concerning a story about Japanese intentions that had appeared in the Washington papers under the dateline "Allied HQ Aust 27 Apr." The story revealed a "naval concentration . . . in [the[ Marshalls, apparently preparing for [a] new opn [operation]." It was a very concerned General Marshall who pointed out that if the Japanese became aware of this story they would be "justified in believing their codes broken -- which would be disasterous [sic]." MacArthur denied any knowledge of a release containing such information.69 Nevertheless, this incident may have contributed to MacArthur's negative attitude toward the use of COMINT in messages to subordinate commanders.

Neither the Japanese, nor apparently the Pacific centers, ever became aware of the episode. Certainly nothing arising in the southwest Pacific interfered with the center's requirements to provide details of Japanese plans to attack Port Moresby. Several vital pieces of information, however, continued to elude the navy's analysts: when did the Japanese plan to attack; what forces were committed; and how many objectives were represented in the organization revealed in translations of 9 and 22 April 1942?

The designators employed -- MO, RZ, RX, RY -- implied an operational concept transcending merely MO/RZ, the acknowledged designators for Moresby. Rochefort's people worked tirelessly to find specific correlations and east the level of anxiety in CINCPAC headquarters. Layton was particularly alarmed by the unknown elements of the Japanese plan and speculated that the impending offensive could lead to a serious interruption in the flow of supplies to Australia and even to an attack on Australia itself.70 This conclusion was stimulated in party by HYPO reports of the 22d and 23d, which for the first time detailed the strength of the Moresby operation.

HYPO's reports were based on translations by HYPO and NEGAT of Japanese Navy messages intercepted between 19 and 22 April and on a report form Melbourne, HYPO's reports revealed that the MO Strike Force included Cruiser Division (CruDiv) 5, CarDiv 5, and possibly submarines, and the light carrier Ryukaku (*).71 An equally alarming report on the 23d referred to the "powerful concentration" in the Truk area and erroneously included the carrier Kaga.72


By 30 April 1942, after Admirals King and Nimitz had completed their first meeting in San Francisco, none of the most vital issues was resolved, although the scope of the Japanese plan was now known. Translations and analytic effort identified RXB as Tulagi and RY as an island in the Giblets far to the east of Moresby. In addition, the names of Deboyne, Samari, and Cape Rodney "Detachments" appeared in translations of Fourth Fleet messages, suggesting that these locations in the Solomons Sea off the southern tip of Papua/New Guinea were included in the MO operation. A growing sense of urgency in Hawaii prompted Layton to publish a warning on the 30th that the Moresby operation "will begin very soon."73

In a surprise move on 30 April, the Japanese fleet undertook to change their locator system. Their attempts to conceal the new system failed completely, however, when Hawaii intercepted a Fourth Fleet message containing both old and new designators. Hawaii translated a CINC 4 message to the Fourth Fleet, CruDiv 5, 11th Air Fleet, 4AAF, 5AAF, CarDiv 5, Kamikawa Maru, Ryukaku[*], a destroyer division (DesDiv), and air tender, an air force at Rabaul, and others, indicating the "enemy" had discovered some or all of the reference point designators. Detailed changes in the system were mandated using the "Irohani Syllabari." The translation conveniently substituted digraphs for trigraphs throughout the operational area.74

This development, which analysts saw as the final step before the beginning of Japanese operational activity, prompted HYPO to publish n evaluation of the status of Japanese preparations. Their assessment of the forces gathered by the Japanese for their attack on Port Moresby was remarkably accurate. The MO, or Moresby, campaign was now under way. It encompassed southeast New Guinea and the Louisiades Archipelago. Australia was not an objective either at that time or in the follow-on operations to the east, preparations for which could not be clearly seen. The Aleutians were suggested, however, as a possible objective. The Japanese forces engaged in the campaign were listed: CarDiv 5 and CruDiv 5, less the Nachi (CA), with CruDiv 8 and Destroyer Squadron (DesRon) 6 available. Also included were Gunboat Division 8 and New Britain Air (5AAF) consisting of Tainan Air, 4th Air Corps, and Yokohama Air. The Commander, CruDiv 5, Vice Admiral T. Takagi, was in command of the Strike Force, and CINC 4, Admiral Inouye, was in overall command. At the time both were located in the Rabaul area. Light forces were said to be en route to the operations area. An unspecified number of air tenders and transports, one Submarine Squadron (SubRon) and the light carrier Ryukaku [*] were also included.75 Melbourne analysts disagreed with some portions of the HYPO translation, but both centers agreed on the fundamental point, that the MO operations was under way.76 (The MO-related forces consisted of almost 300 vessels of all types, including the following: three CVs, nine CAs, fourteen DDs, eight SSs, and thirty-three warships of other types. Lundstrom says that the total number was 282.) CINCPAC's estimate published in his OP Order listed a force considerably larger and significantly different from that published by HYPO but still far short of the actual strength. The discrepancy between Layton and


Rochefort probably indicated the inclusion of units Rochefort could not confirm. Though specific types of vessels were listed, CINCPAC did not attempt to correlate them with an Order of Battle: one BB from Battleship Division (BatDiv) 3, three CAs, three aircraft ferries, and parts of four CruDivs (4, 5, 6, and 18), sixteen DDs, and twelve SSs.77 Altogether the actual task force represented the largest yet assembled anywhere in the Pacific.78

In addition to intramural disagreements, during 1-5 May analysts at all three centers were victimized by massive Japanese communications security measures introduced throughout the Combined Fleet. In the Fourth Fleet the use of tactical callsigns by ships, a change of all shore station callsigns, the use of false callsigns by ships in the Moresby invasion forces, and a marked increase in the volume of high precedence communications complicated temporarily the task of identifying the fleet's communications infrastructure.79 HYPO was probably not alone among the centers in admitting to "some confusion" caused by the communications deception plans introduced on 4 May into those elements of the Combined Fleet already preparing for their Midway operation. The most deceptive part of the plan was implemented by the Fifth Fleet on 3 May to cover the drawdowns on forces in home waters necessitated by the Moresby and Midway buildups. Such enforced removals left exposed the eastern approaches to Japan. New callsigns were introduced in a special communications drill intended to give the appearance of large forces where none actually existed.80

In Strike Force-related communications, Japanese communications security measures may also explain what Melbourne intercepted on 4 May concerning the appearance of the Kaga (CarDiv 1), CarDiv 2, Hiryu, Soryu, and elements of BatDiv 3 and CruDiv 8 at Truk in an anchorage assignment message. No dates for the arrivals were given, but a second translation from Melbourne seemed to reinforce the likelihood of the Kaga appearing at Truk when "her repairs" had been completed.81 There are a number of explanations for the messages concerning the Kaga, CarDiv 2, etc., appearing at this time. Whatever might have been the real reason for the messages, they added momentarily to the confusion by rekindling the mistaken idea among U.S. analysts that this activity was in some way related to the Moresby operation.

The Japanese security measures affected American cryptanalysis and traffic analysis and slowed the development of two major discoveries: the role of the Strike Force in the invasion and the impact on X-day of bad weather delaying the progress of the Strike Force from Truk to the Coral Sea. On 2 May 1942, both Hawaii and Melbourne intercepted one and possibly two messages for the Strike Force detailing this information and much more. One message concerned the delivery of carrier aircraft to Rabaul by CarDiv 5 and the fact that bad weather was interfering with flight operations. The resulting delays caused a postponement of the Strike Force's schedule until 8 May (7 May on the U.S. calendar).82

On 3 May, Melbourne cryptanalysts noted that a possible second message to the carriers implied that the Strike Force planned to pass north-northeast of Bougainville before its aircraft bombed Tulagi.83 The same message, according to HYPO, ordered the Strike Force to bomb Moresby on X-2 and X-3. Based on these messages, HYPO concluded


Japanese Warship Hiryu

Japanese Warship Soryu

that X-Day was 10 May. It was not until the fifth, however, after much activity between HYPO and the other two centers in technical channels, that these translations were finally resolved and CINCPAC was able to advise the task forces of Japanese intentions:

CINCPAC 050329Z to CTF16 and 17: Jap Commander Moresby Striking Force (ComCrudiv5) indicates 4 May that if Blue Striking Force is determined in Coral Sea (questionable location)_ Orange Striking force will proceed North North East of Bougainville thence to southward. At 0600 Item 5 May after arrival unknown place will proceed accordance further orders. If no further orders will go to Tulagi. If plane search in southern and another sector needed Carrier Div 5 to send his bombers to Tulagi at daybreak.84

CINCPAC 050345Z to CTF16 and 17: Reliable indications of 3 May: Orange Moresby Striking Force composed of Crudiv5 and Cardiv5 will launch attacks on Allied bases Port Moresby area on Xray minus 3 or Xray minus 2 days. Attacks to be launched from Southeast (fairly good but not certain). Xray day not known but one indications points to 10 May as Xray day. Above attacks to be carried out until successful completion by Orange.85


Meanwhile, MacArthur notified Admiral Fletcher concerning the forces he was likely to encounter at Moresby. Specifically, throughout the day on 4 May MacArthur sent messages summarizing enemy activity at Tulagi, Savo, and Guadalcanal, and especially a report of an aircraft carrier and a possible battleship/cruiser in position west of Bougainville; all this information was based on aerial reconnaissance.86 messages based on aerial reconnaissance usually gave the CTFs the locations as well as the types of Japanese vessels sighted.

By 5 May, the centers were once again reporting on translations made the same day. Derived from Japanese messages on 5 May was the fact that an "Occupation Force" (a) was still in the Coral Sea*; (b) would not be complete until joined by the Tulagi Force at 1400 on 6 May 1942; and 9c) would not leave the Coral Sea* before 1800 on the 7th when it "moved south of Emerald."87 (*What was probably meant by the Japanese in these instances was Solomons Sea, the body of water between Bougainville and Papua/New Guinea, where the ships were actually located.) In addition, General MacArthur informed Admiral Fletcher and Admiral Nimitz that reconnaissance aircraft from Australia reported sighting a Japanese carrier, a cruiser, and a tender in the western Solomons Sea. The locations given were west not east of Bougainville, indicating the carrier was not part of the Strike Force.

MacArthur's headquarters also published a translation supporting the strong probability that the carrier and its escorts were part of the "Occupation Force." The 5 May Japanese Navy message gave a location for the Occupation Force that corresponded exactly with that of the carrier in the reconnaissance reports. late on the fifth, in a rare departure from l practice, MacArthur's headquarters sent Fletcher and Nimitz a translation from the Melbourne RI center that stated that at "0600 5 May the MO Occupation Force" will be in position "8 degrees north, 155 [degrees] blank minutes east speed 23 course 300."88 Moreover, COMINT revealed that these ships did not plan to leave the Solomons Sea until 78 May after they were joined by the Tulagi Force.89

Melbourne also published translations concerning Japanese reconnaissance flights over the Coral Sea by 5AAF fighters and called attention to the use of Rabaul radio as a tactical headquarters by Admiral Inouye and possibly other commanders.90 Other translations revealed the scope of Admiral Inouye's responsibilities regarding the forces involved in Moresby and the entire region northeast of Australia.91 These items were not considered important enough by either Nimitz or MacArthur to be forwarded to the task forces at that time.

On 6 May MacArthur's reconnaissance aircraft filled the sky with activity over the vulnerable Japanese convoys awaiting the appearance of the Striking Force. The Japanese Invasion/Occupation, Covering, and Support Forces, including the carrier Shoho, circled between Deboyne and Bougainville, where they were repeatedly spotted by American reconnaissance aircraft and attacked by B17s from Moresby. none of this activity was reflected in the communications of Japanese air patrols reportedly in the


same area, suggesting that Japanese aviators were avoiding contact with the American patrols.92

Throughout the 5th and into the 6th, MacArthur repeatedly sent B-17s to bomb the Japanese convoys. Each sighting and each attack was dutifully reported to the American task force commanders along with the location and makeup of each group of ships.93 In the message dispatched at 060145Z, MacArthur's headquarters also notified the task force commanders that, from COMINT intercepted after 2230Z on the 5th, he learned that the Japanese had sighted a U.S. carrier and cruisers "on course 190 speed 20."94 This message, which lacked a meaningful reference point and was either highly inaccurate or poorly decrypted/translated, represented the first indication that the Japanese knew where the American carriers were located.

As dawn came on 7 may, the two American task force commanders knew far more than their Japanese counterparts about the plans and dispositions of the opposing forces. Through a combination of communications intelligence and aerial reconnaissance, they knew that a single Japanese carrier and its escorts had actually been sighted west of Bougainville and that the Japanese convoys carrying troops for the invasion of Moresby and their protective covering forces were also loitering in the Solomons Sea west of Bougainville.

From communications intelligence alone, they knew that the Japanese were devoting three carriers to the Moresby operation, the Shoho, the Shokaku, and the Zuikaku; the Invasion Force covered by the Shoho planned to enter the Coral Sea around the southern end of Papua/New Guinea after capturing the island of Deboyne, Samarai, and other potential seaplane bases in the Louisiades; the two Strike Force carriers of CarDiv 5, the Shokaku and the Zuikaku, were passing north-northeast of Bougainville probably to enter the Coral Sea south of Tulagi; the Strike Force intended to approach from the southeast and bomb Moresby on X-3 and X-2; and that X-day was probably 10 May.

The Battle Begins

The Battle of the Coral Sea began on X-3 day, 7 May 1942. At 0815 local time, a Yorktown reconnaissance pilot informed Admiral Fletcher of two carriers and a Japanese task force at 10 degrees south, 152 degrees 27 minutes east. This location, where the Solomons Sea and the Coral Sea meet, was consistent with the reports from communications intelligence and sighting reports from MacArthur's aircraft. Because of improper encoding, the reconnaissance report was inconsistent with intelligence in identifying two carriers. A little over an hour later, Fletcher launched a full air strike from both the Yorktown and the Lexington against what he had every reason to believe was the Shoho escorting the Moresby Occupation Force leaving the Solomons Sea. At approximately 1136 local time, Admiral Fletcher was undoubtedly gratified to receive that brief but famous message from Lieutenant Commander R.E. Dixon, a dive-bomber squadron commander from the Lexington: "Scratch one Flattop."


During the next few hours, Admiral Fletcher and Fitch received the bulk of their COMINT support from radio intercept detachments placed aboard their carriers by HYPO.95 Admiral Fitch was also supported by a detachment in which the linguist was the late Admiral Ranson Fullenwider, who left on record of his experiences during these historic events. The Yorktown detachment closely monitored the efforts of CarDiv 5 to recover aircraft from the carriers Shoho. They followed closely the Japanese Strike Force's attack against the oiler Neosho and its escort, the destroyer Simms. The linguist has reported that he used COMINT to influence Admiral Fletcher not to risk discovery of his task force by breaking radio silence to either warn or recall the two vessels.96 CTF 16 was a considerable fighting force quite able to defend itself. There may be another explanation for this dubious decision.

At 1749 local time, the Yorktown's radar detected approaching enemy aircraft about twenty-five miles away. Fletcher launched fighter protection immediately, and the American aircraft quickly found the approaching Japanese. About thirty minutes later, the Japanese abandoned all radio security, and the detachment reported to Fletcher a message to Commander, CarDiv 5, that "his attack squadron has been annihilated by enemy fighters."97

Before dawn on 8 May, HYPO provided CTFs 16 and 17 the new callsigns, frequencies, and procedures being used by the Japanese Strike Force and gave a new location for the Japanese carriers. They were then located only slightly to the northeast of the American task forces.98Within three hours of receiving the message, Admiral Fletcher's planes, searching to the east as directed, found the enemy carriers. Shortly afterward, Admiral Fletcher radioed both Nimitz and MacArthur that a Japanese naval force consisting of "2CV, 4CA and many DD" was located at "Lat 12 Long 156." Fletcher gave his own position as "[Lat] 14-30 Long 154-30."99

The attack on the Yorktown and the Lexington lasted a little less than an hour and a half, from 1113 to 1240 local time. Immediately after the attack, as happened the day before, the Japanese pilots and radio operators aboard the carriers "opened up their radios" and discarded security considerations. it was apparent immediately that something had happened to the Shokaku when it failed to respond to calls from its aircraft and the Zuikaku began sending homing signals and recovering the Shokaku's planes. The intercept operators aboard the ,i>Yorktown listened to these efforts and reported throughout the afternoon and evening that many aircraft were lost at sea or landed on isolated island beaches. After the Yorktown air groups completed their attacks, intercept confirmed for Fletcher the damage reports he was receiving from his aviators.100 At 1237 local time, Fletcher notified Nimitz and MacArthur that he had damaged an enemy carrier with two 1,000-pound bombs and two torpedo hits.101 He also reported that his own force had sustained some damage.102 This message was probably welcome in Hawaii since a few minutes earlier the Melbourne center had sent them a message indicating a U.S. carrier had been sunk. A message from the Zuikaku, intercepted by HYPO at about the same time contained even more ominous news. It aid that one U.S. carrier had been sunk and another had sustained three sure direct hits.103 Both centers combined to place CINCPAC in


the remarkable position of learning from Japanese sources the extent of damages to his forces before he received the damage reports from his own task force commander.

Fletcher and Inouye sere apparently unable to assimilate and evaluate the unique and voluminous reports both undoubtedly received from COMINT and other sources about plane losses and carrier damage sustained by their enemy counterpart. Accordingly, each chose similar courses of action late in the afternoon of 8 May: each broke contact with the enemy and retired from the scene. Fletcher advised Nimitz of his plans to retire overnight and "fill Yorktown complement planes as far as possible from Lexington and send that ship to Pearl."104 Sensing that his intentions to retire might be misunderstood, Fletcher also advised Nimitz and MacArthur that "another enemy carrier has joined enemy force."105

Similarly, Admiral Inouye, aware of his own losses, particularly in aircraft of all types and pilots, ordered the Strike Force to break contact. He also postponed the attack on port Moresby. Melbourne intercepted the postponement order and reported it the same day.106 Yamamoto abruptly rescinded Inouye's order to break contact and ordered Admiral Takagi, Commander, CruDiv 5, to find and destroy the remaining U.S. fleet. In less than a full day, however, the search was dropped. The tight Japanese schedule for post-Moresby operations took control of events.107


The Battle of the Coral Sea was unique in U.S. naval annals and was the scene of many cryptologic triumphs. To OP-20-G and the radio intelligence centers in Hawaii, Corregidor, and Australia, the battle and its preliminary sparring afforded for the very first time the opportunity to support three major shore-based headquarters, Washington, Pearl Harbor, and Melbourne, each vitally concerned with both the conduct and the outcome of a major sea battle. By providing accurate and timely warnings of Japanese intentions beginning as early as January 1942, COMINT enabled Admiral King and Admiral Nimitz to position scarce U.S. Navy carriers task forces where they could interrupt and frustrate Japanese plans and intentions. Moreover, after the Japanese Navy's General-Purpose Code (NH 25) became readable in march 1942, COMINT provided invaluable information concerning the Japanese timetable and order of battle for the invasion of Port Moresby up to the very eve of the battle.

The battle and its preliminaries were also unique to OP-20-G at the fighting echelons. Special intercept/linguist detachments, the idea for which can be traced to the U.S. Asiatic Fleet's attempts to monitor Japanese communications during the Imperial Naval Exercises of the 1930s, were placed on board the aircraft carriers Enterprise, Lexington, and Yorktown in early 1942.108 These detachments provided tactical support to the task force commander during extended carrier sorties in the regions between Australia, Japan, and Hawaii.

When in contact with the enemy, the detachment intercepted any HF communications related to the action. On 7 and 8 Ma particular, the RI units provided an invaluable


service to the commander afloat by following closely the communications between Japanese carriers and their aircraft. When not occupied with periods of actual fighting, they helped to explain COMINT based reports sent to the task forces by their headquarters in Washington, Hawaii, and Melbourne. This vital service closed a critical COMINT loop by linking the strategic system ashore and the tactical support system at sea. By all accounts, however, the detachment/command relationship cannot be said to have functioned smoothly at this stage of the war.

It is important to remember, however, that U.S. carriers observed a strict regimen of radio silence until located by the opposing force. This policy precluded pre-hostilities information of any kind passing from the task force to either CINCPAC or COMSOWESPAC headquarters. With minor exceptions, therefore, even after the Japanese discovered the American carriers, the commands ashore did not benefit from the local knowledge of the task force commander.109

There is no indication that RI units afloat prepared any reports for consumption beyond the limits of their respective flag plots. This conclusion is reinforced by the fact that reports from the shore-based centers did not contain details of the battle revealed by the RI units. Where there was duplication, it was clearly based on independent intercept, such as ship-to-shore communications or sighting reports sent by Japanese aircraft or relayed by Japanese ground stations that were heard by both the detachment(s) and HYPO/Melbourne. It is also clear that, if the shore-based centers were to support their respective commands with details of a battle in progress, further refinements of the instructions to the RI units afloat were required. The time remaining before the next crisis, however, did not permit the system to develop any corrective measures.

Immediately after the battle, Admiral King accused Admiral Fletcher of an "apparent lack of aggressive tactics," i.e., failure to launch night attacks. This criticism was echoed by Biard, when he described Admiral Fletcher as confused about Japanese intentions and dispositions.110 Both Nimitz and John Lundstrom, author of The First South Pacific Campaign: Pacific Fleet Strategy December 1941-June 1942, placed of the blame on communications intelligence. Lundstrom specifically noted in an April 1983 article in Cryptolog that COMINT failed Admiral Fletcher, who, as a result, "placed his Task Force 17 in jeopardy" on the morning of 6 May 1942.

In his letter to Admiral King, Admiral Nimitz excused Fletcher's "long delay and apparent lack of aggressive tactics." He said that these failings "can be charged to the lack of sufficiently reliable combat intelligence."111 If this was a criticism of Biard, it was probably unwarranted. His after-the-fact log indicated that the Japanese radios were copied as soon as they broke radio silence the morning of 7 May. Biard, according to his account, was also aware that Hawaii and Melbourne sent Fletcher regular intelligence advisories prior to the battle. His monograph, while discussing in critical terms the disposition of Admiral Crace's task force of Australian and American warships, actually indicated that on 7 May 1942 Fletcher had in fact directed his search aircraft east of the Louisiades where COMINT had foretold, and aerial reconnaissance had already located,


Japanese transports, a carrier, and other warships that represented a threat to Crace. (In Biard's article in Cryptolog, Admiral Crace's name is consistently misspelled.)

With reference to the situation summary for 6 May, outlined above, it is difficult to imagine what more the combat intelligence centers might have contributed concerning the Japanese forces prior to the engagement. Though Lundstrom was unaware of its existence, Biard's log indicates that the RI detachments were most supportive of Admiral Fletcher during the two days of actual fighting. in the context of the total intelligence picture available to Admiral Fletcher, there may never be another situation in which a single source of information proves more supportive. Moreover, it is equally difficult to imagine a situation in which two intelligence sources proved more complementary than COMINT and aerial reconnaissance were on 5 and 6 May 1942.

The Land Route to Port Moresby

The Japanese stubbornly refused to conceded that this attempt on Moresby would be their last. Between 9 and 19 May, the Japanese continued to manifest an interest in reopening the original Moresby scenario. On the 19th, in fact, Layton reported that 15 June had again been indicated as the date of resumption. On the same day Melbourne published a message from Tokyo Naval Intelligence, dated 18 May 1942, which, though unremarked by the other centers, acknowledged that "enemy" aircraft could no prevent Japanese resupply of Moresby (once taken) and proposed construction of a land route from Lae.

The feasibility of the idea was recommended for study, and for nine days Melbourne continued to publish translations on this subject. On 27 May, in fact, a message dated the 23d was published containing a detailed route across the Owen Stanley mountains proposed by the 8th Base Force, Rabaul. This message referred to "General Staff Serial 507," suggesting very high-level Japanese interest in the idea of a land route possibly to both capture and resupply Port Moresby.112 Such ideas die hard, especially when they gain the favor of high-level planning staffs, but immediately after the Battle of the Coral Sea, other plans dominated the affairs of the Japanese Army and Navy.113


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[See the Bibliography for complete citations to the items listed below. For example: "Morison" refers to "Morison, Samuel Eliot. History of U.S. Naval Operations in World War II. (Boston: Little, Brown and Company)"]

2. Hattori Takushiro, The Complete History of the Greater East Asia War, Masu Publishing Co., 1953, Vol. II, 101. Hereafter: Hattori.

3. From a translation of this order found in the King papers at the Navy Historical Center and generously called to my attention by a fellow researcher, Captain Allen Bath, USN (ret.).

4. H.P. Willmott, The Barrier and the Javelin, Annapolis, Maryland: Naval Institute Press, 1983, 3. Hereafter: Willmott.

5. Hattori, Vol. II, Part 1, 113-15.

6. Hattori, Vol. II, Part 1, 100-01, 114-15, and 123.

7. It was from the airfield at Lae that Amelia Earhart began the last and fateful leg of her around-the-world flight in 1937.

8. This description of Japanese plans is based on the following: Hattori; Fuchida Mitsuo and Okumiya Masatake, Midway, The Battle that Doomed Japan, Annapolis, Maryland: U.S. Naval Institute, 1955. Hereafter: Fuchida; John B. Lundstrom, The First South Pacific Campaign; Pacific Fleet Strategy December 1941-June 1942, Annapolis, Maryland: Naval Institute Press, 1956. Hereafter: Lundstrom; and Samuel Eliot Morison, History of U.S. Naval Operations in WWII, Boston: Little, Brown and Co., 1971. Hereafter: Morison.

Fuchida and Hattori describe a process of compromise beginning in January 1942 both within the navy and between the navy and the army that caused the Imperial General Staff's prewar policy of a strategic defense to come under attack, primarily by planners in the staff of the Combined Fleet. See Fuchida, 48-63, and Hattori, Vol. II, Part II, 100-49. See also Lundstrom, 23-25 and 40-41, who follows the Japanese accounts though not entirely from the same sources. Morison, Vol. II, 2, noted that a strategy that relieved the army of any large-scale commitment of forces had actually been formulated in 1941 and published in Fleet Op Order #1, in November 1941. This contention is not supported by the Combined Fleet translation cited above or by Japanese accounts.

9. Lundstrom, 27.

10. Hattori, Vol. II, 124.

11. Ibid.

12. Lundstrom, 39-40.

13. Morison, Vol. III, 259-60.

14. See, for example, his message of 2 January 1942 to CINCPAC that urged an "expedition of raid character against enemy bases in Gilbert Islands. . . ." Blue Flag Messages, Reel TS1, Naval Historical Center, Operational Archives.

15. Morison, Vol. III, 253-64.

16. Morison, Vol. III, 267; and Nimitz Command Summary, 1941-9142 NSA History Collection IV W II.2.2. Hereafter: Nimitz Command Summary. The Nimitz Command Summary for 25 February 1942 contained a Japanese message to Berlin that mentioned that "Japan will be unable to sustain losses [in pilots and aircraft] at their present rate."

17. Morison, Vol. III, 387-89.

18. Lundstrom, 40-41.

19. Lundstrom, 46; SRH012, RG457, NA.

20. It was the Shoho's fate to be known in COMINT by the wrong name throughout its brief existence. Called the Ryukaku as a result of cryptanalytic and translation errors, its correct name was revealed by a POW in late 1942. Hereafter, whenever the incorrect name appears, it will be followed by an asterisk(*).

21. SRMN012, COM 14 TI Summaries with comments by CINCPAC Intelligence and Warplanes Sections; RG457, NA. To distinguish between the output of COM 14 and CINCPAC Intelligence, I use the letters H for COM 14 and L(Layton) for CINCPAC. SRMN012/H, 17 March 1942, repeated Corregidor's unlocated report of this event.

22. Lundstrom, 66.

23. Morison, Vol. IV.

24. See Com 14 020344Z May 1942, in CINCPAC Message Files, RG38, NA.


25. Three groups were formed to implement the Directive: MO Striking Force, MO Invasion Force, and Tulagi Invasion Force. South Seas Op Order #13 also specified "Cooperating Forces" in the form of Base Air Forces, and an AEF, (i.e. submarines in an Advanced Expeditionary Force), along with a timetable of events. The timetable was as follows:

The final section of OP Order #13 gave a T/O (Table of Organization and Functions): MO Striking Force (CarDiv 5); The MO Striking Force was to cover the Invasion Force, seek and destroy allied naval force, attack allied air bases in Australia and Moresby; MO Invasion Force: The force consisted of troopships, the Shoho and four CAs; and the Tulagi Invasion Force. The Tulagi force was to break up and join the MO and RY Forces after Tulagi was seized. Lundstrom, 68. RY meant Ocean and Nauru.

26. History of OP-20-GYP, Classified Naval Archives, Crane Indiana. Hereafter History of OP-20-GYP.

27. Fabian interview, NSA Oral History #09-83.

28. SRMN012, RG457, NA.

29. See SRMN012/H and L for the period 17-19 February 1942. RG457, NA.

30. See, for example, COMINCH message 021718Z January 1942, in Blue Flag messages, Reel #TS-1, Naval Historical Center, Operational Archive.

31. Rear Admiral Edwin T. Layton, with Captain Roger Pineau and John Costello, And I Was There, (New York: William Morrow and Co., 1985), 357.

32. Ibid.

33. Dyer Interview; U.S. Naval Institute, 224.

34. History of OP-20-GI, NSA History Collection, IVWI.5.3, Hereafter History of OP-20-GI.

35. Ibid.

36. History of OP-20-GI; SRH012, RG457, NA.

37. SRMN012(L) 29 January 1942, RG457, NA.

38. SRMN012(L), 24 February 1942, RG457, NA.

39. SRMN008, RG457, NA.

40. SRMN004, RG457, NA.

41. See, for example, Bulletins number 46 and 47, 300131Z April 42 and 010311Z May 1942, respectively.

42. SRMN008, RG457, NA.

43. SRMN012, RG457, NA. Layton, 378-79, identifies himself as the author of the Bulletins and mentions some of these sources.

44. See the Operational Priority messages from Com 14 to CTF 16, 070630Z and 071852Z May 1942 in the CINCPAC Message Files, RG38, NA.

45. The record referred to here in the CINCPAC Message File.

46. SRNS1517, 20 March 1942, Fleet Radio Unit Melbourne (Fleet) Daily Digest, 20 March 1942-30 October 1944, RG457, NA.

47. See NSA OH 09-83, Captain Rudolph T. Fabian, USN (ret.)

48. Denniston papers, NSA History Collection. Although Denniston used the word cryptography, what he meant was cryptanalysis.

49. SRH355, RG457, NA. Hereafter Holtwick.

50. History of OP-20-GYP-1.

51. Ibid.

52. Layton, 376.

53. History of OP-20-GYP-1.

54. Dyer interview.


55. According to the Clarke Papers in the NSA History Collection, the British, finding it completely counterproductive, abandoned this practice during the mid- to late thirties.

56. Layton.

57. SRMN012/H, RG457, NA, 2 April 1942.

58. SRH012, RG457, NA, 3 April 1942.

59. One was reported (AS), which analysts believed stood for San Francisco. Com 14 to COMB 0161948Z May 1942, CINCPAC Message File.

60. SRNM0123/0126, 23 March 1942. Miscellaneous records pertaining to Japanese Naval Communications WWII, RG457, NA. Hereafter, only the SRNM number and date will be given.

61. CV, CarDiv1.

62. SRH012, 7 April 1942, SRNM0381, 8 April 1942, SRNS1517, 9 April 1942. Fleet Radio Unit Melbourne (7th Fleet) Daily Digests 20 March 1942-31 October 1944. Hereafter, only the SRNS number and date will be given. All are RG457, NA.

63. SRN913, RG457, NA, 9 April 1942. This message revealed the existence of an MO striking force, i.e., carriers; MO Attack Force; RZP Occupation Force; RZP Support Force; RXB Occupation Force; and RY Occupation Force. RXB and RY were not immediately identified.

64. SRMN012/L, RG457, NA, 11 April 1942.

65. D.M. Horner, "Special Intelligence in the Southwest Pacific Area in WWII." Australian Outlook Journal of the AIIA, December 1978, Vol. 3, No. 3; Willoughby Papers, U.S. Army Military History Institute, courtesy Dr. E. Drea.

66. Ibid.

67. The limits of MacArthur's air operations in early May were detailed in part one of a 15 May 1942 message from COMINCH to CINCPAC, 142100-142118Z May 1942. CINCPAC Message File, RG38, NA.

68. Ibid.

69. Ibid.

70. SRMN012/L, RG457, NA, 23 April 1942.

71. SRNM0509H, SRNS1517, 10 April 1942; SRNM0613/N 20 April 1942 and SRMN012/H, 22 April 1942; all RG457, NA.

72. SRNM0559/H, RG457, NA, 23 April 1942.

73. SRNM0687, 20 April 1942; SRH012; SRNS1517; CINCPAC Enemy Activities File, SRH272/H; SRMN012/L; all 30 April 1942 and all RG457, NA.

74. SRNM0712, 30 April 1942, RG457, NA.

75. SRNM0713 and SRMN012, 1 May 1942, RG457, NA.

76. SRNS1517, 1 May 1942, RG457, NA.

77. NSA History Collection IV W VII.18.

78. Morison, Vol. IV, 21-28: The Tulagi Invasion Force sailed from Rabaul on 30 April 1942. Willmott: The Strike and Covering Forces sailed from Truk on 1 May. Lundstrom: CINC 4 changed his headquarters from Truk to Rabaul on 1 May 1942.

79. SRMN012, 30 April 1942 and 1 May 1942; SRNS1517, 1 May 1942, all RG457, NA.

80. SRH012, 3 May 1942, and SRMN012/L, 5 May 1942, RG457, NA.

81. SRNS1517, 4 May 1942, RG457, NA.

82. SRNM0911/N, 2 May 1942, RG457, NA.

83. SRNS1517, 4 May 1942, RG457, NA.

84. CINCPAC Message File, RG38, NA; also SRH012, 3 May, and SRNS1517, 4 May 1942; both RG457, NA.

85. CINCPAC Message File, RG38, NA. See also Belconnen (Melbourne 050150Z, same file.

86. In the CINCPAC Message File, RG38, NA, see the following: 040236Z, COMSOWESPACFOR to CTFs; 040950Z COMSOWESPACFOR to all CTFs; 040159Z COMSOWESPACFOR to CTFs.

87. SRNS1517, SRMN012, 5 May 1942, RG457, NA.


88. COMSOWESPACFOR to CTF 16, CTF 17, CINCPAC, etc. 051200Z May, CINCPAC Message File, RG38, NA.

89. SRNS1517, SRMN012, 5 May 1942, RG457, NA.

90. CINC 4 removed his headquarters from Truk to Rabaul on 1 May 1942. Lundstrom.

91. SRNS1517, SRMN012, 5 May 1942; RG457, NA.

92. SRNS1517, 6 May 1942, RG457, NA, contained a translation of "5AAF Battle Report No. 26" for 5 May that reported on the patrol of "9 type O fighters."

93. MacArthur to all CTFs 060025Z, 060107Z, 060145Z, 060246Z, 060754Z, 060925Z, 061226Z.

94. All of these messages were found in the CINCPAC Message File, RG38, NA.

95. Monograph by Forrest R. Biard, Captain, USN (ret), Winter 1989, Vol. 10, No. 2, a special issue of Cryptolog, a publication of the Naval Cryptologic Veterans Association (NCVA). Biard was a linguist for the detachment serving Admiral Fletcher. Hereafter Biard Monograph.

96. Ibid.

97. Ibid.

98. HYPO to CTFs 16 and 17, 071825Z May 1942, CINCPAC Message File, RG38, NA.

99. CTF 17 072228Z, May 1942, CINCPAC Message File, RG38, NA.

100. Biard monograph.

101. CTF 17 080137Z, May 1942, CINCPAC Message File, RG38. NA.

102. Ibid.

103. Belconnen to COMB 080110Z May 1942, CINCPAC Message File, RG38, NA; SRNM0888/N, 1129 local time 8 May 1942; SRN897/H 1100 local time 8 May 1942; SRNM0908/H, Rabaul report of 8 May 1942; all RG457, NA.

104. CTF 17 080252Z, May 1942, CINCPAC Message File, RG38, NA.

105. CTF 17 080332Z and 080348Z, May 1942, CINCPAC Message File, RG38, NA.

106. SRNS1517, 8 May 1942, RG457, NA.

107. SRNS1517, 8 May 1942; SRNM0865/0999, 0908, 0910, all 8 May 1942; SRH272, SRNM0909 8 May 1942; al RG457, NA; Fuchida.

108. HC IV W XI.14; SRH289, RG457, NA.

109. A joint CINCPAC/COMSOWESPACFOR post-operation evaluation (CINCPAC 200359Z, May 1942, CINCPAC Message File, RG38, NA) indicated that all task forces maintained strict radio silence until their presence was discovered by the enemy. The Nimitz Command Summary and the CINCPAC Message File reflect post-discovery messages from task force commanders to a wide audience. They were not intelligence reports, however.

110. Biard monograph.

111. Nimitz to King, 29 May 1942, King Collection, U.S. Navy Historical Center, Operational Archives.

112. SRMN012/L, SRNS1517, 19 May 1942, SRNS1517, 20 May, 27 May, and 28 May 1942, all RG457, NA.

113. For further information on Japanese preparations for an overland assault on Port Moresby, see the work of Dr. Edward J. Drea in MacArthur's Ultra: Codebreaking and the War Against Japan, (Lawrence: University of Kansas Press, 1991.)

Transcribed and formatted by Patrick Clancey